I. Field of Invention
This invention relates to a process for making pentachloronitrobenzene by the nitration of pentachlorobenzene. In particular, this invention relates to a process whereby highly pure pentachloronitrobenzene is produced. And, especially, this invention relates to a process for making pentachloronitrobenzene wherein the resultant levels of pentachlorobenzene and hexachlorobenzene impurities are extremely low.
II. Description of the Prior Art
Pentachloronitrobenzene (sometimes referred to herein as PCNB) is widely used today as a soil fungicide for many farming applications. While this use has shown many beneficial results, the present commercial products of PCNB have been questioned on environmental grounds because of the presence of two contaminants contained therein. These chemicals, pentachlorobenzene (sometimes referred to herein as PENTA) and hexachlorobenzene (sometimes referred to herein as HCB), have been found to bioaccumulate in the fatty tissue of animals. Therefore, their presence in this desirable soil fungicide, even in relative minor amounts, could cause a health hazard. For example, foodstuffs produced from soil which has been treated with quantities of impure fungicide may have PENTA and HCB leached into them and, thus, these impurities may accumulate in humans when they are eaten. Also, cattle and other livestock that graze on treated grass or ther pastures or grains may accumulate undesirable amounts of these impurities. And, furthermore, the farmer when applying PCNB to the soil may breathe in significant quantities of these impurities.
PCNB produced by prior art commercial processes usually contains at least about 0.5 to 2.0% or greater by weight each of PENTA and HCB. Although these percentages may appear to be minor amounts, they are still high enough to raise an increasingly stronger environmental concern by the manufacturers, consumers and the governmental regulatory agencies. In fact, a commercial PCNB product with less than about 0.2% by weight of either of these contaminants appears likely to be necessary in the future. However, since PENTA and HCB cannot be easily or economically separated from PCNB on large-scale commercial levels, an urgent need exists to develop a process for making PCNB in which the levels of these two other chemicals are minimized below what has previously been reached, preferably below 0.2% by weight level which may be required in the future.
In particular, commercial prior art processes for making PCNB have either chlorinated a chloronitrobenzene (see U.S. Pat. No. 3,026,358, issued on Mar. 20, 1962 to E. A. Lojewski) or have nitrated pentachlorobenzene with substantially pure nitric acid or a mixture of nitric and sulfuric acids. However, all of these processes have suffered from the high impurity problem cited above.